PIC 10A

Final Review

Final exam

• Thursday, June 11, 2015 11:30 AM - 2:30 PM MS 5200. In our usual class room. (Verify on my.ucla.edu!!)

• The final exam is worth 30% of your grade, same weight as 2 midterms. Could be 50% if grading option 2 turns out better for you.

• Length is also roughly 2 midterms (~8 questions with parts).

• Very similar in style to 2nd midterm.

• Exam covers Ch. 1-7 + Sec 9.1, 14.4

• Remember to bring your student ID.

How to study for the exam

Read the slides.

Work through as many practice exams and problems as you can.

See what you missed on midterms.

Look at your notes and examples your TA did.

Review homework assignments.

Read the book.

Seems like a lot of work? IT IS! Get going.

Syntax

Knowing the syntax also helps you write correct structure foryour program.

Knowing the syntax reinforces other concepts.

If you are not confused about syntax the problem becomesclearer.

There is no excuse for not knowing the syntax. All it takes issome memorization.

I/O

Know which way the arrows go with cout and cin!

To read a whole line of input into a string usegetline(cin,my_string);

<iomanip> stuffcout << fixed; cout << setprecision(5) << number;cout << setw(4) << col1 << setw(4) << col2;

You should know about cin buffer and fail state. Recall how we usedcin in a while loop.

Good problem to check your knowledge: Homework 2.

Data types and arithmetic

Example: Casting / Integer division

int a=5;int b=7;double result = (double)(a)/b;

The only unusual arithmetic operator is %. Be sure you understandit and know how to use it.

HW #1 should be crystal clear to you now.

Know all the shorthand notation for assignment and incrementation.+=, counter++, etc.

Good problem to check your knowledge: Midterm 1, problem 1

Strings

Remember to #include <string>

Concatination: Use a + to append one string to the end of another.

Memorize all the string member functions we talked about and makesure you understand how to use them.

Good problem to check your knowledge: Midterm 1, problem 5.

Control structures

You should definitely know the syntax for all control structures.

You should be able to convert one type of loop to another.

You should know when to use a for loop and when to use a whileloop.

You should be very comfortable with &&, || type expressions.

You should understand booleans.

Good homework to look at to review this stuff is the blackjackprogram

Know the syntax for function prototype, function definition and forcalling a function.

It is quite likely that when I ask you to write code that does some taskI will ask you to write a function that does the task.

You should know what a void function is.

You should know how to write a function returning type bool andknow how and when to use such a function.

You should know the difference between passing by value andpassing by reference.

Work through some examples where you chase values of variableslike we did in class.

Functions

Write a function called divide_by_2 that takes as an argument a vector of ints andreturns a vector where all even elements are divided by 2. If the vector is:

5 2 8 3 12

The returned vector should be:

5 1 4 3 6

vector<int> divide_by_2(vector<int> v){

for(int i=0; i<v.size(); i++){

if (v[i] %2 ==0)v[i] /=2;

}return v;

}

Function definition

Just take the first line of the function definition and add a ; to the end.

#include<vector>using namespace std;vector<int> divide_by_2(vector<int> v);

Function prototype

Never put any types in function call!!

Function call only has names of variables.

vector<int> initial;

// Somehow we fill initial vector

vector<int> final = divide_by_2(initial);

Calling a function

You should know the scope rules that we discussed in class.

What is a life span of a variable?

What does local scope mean?

More “inner” variables always supersede the “outer” variables.

Know what global variables are and how to declare them and whenthey are appropriate.

Understand what happens if you declare a variable inside a loop.

Review the “massive scope example”.

Scope

• You should thoroughly understand the difference between the two.

• You should also know when to pass by reference:

• Passing a class to a function

• Passing a vector to a function

Example: Swap function

void swap(int &a, int &b){

int temp;temp = a;a=b;b=temp;

}

Passing by value vs. Passing by reference

Search through a vector and return the first position where valueoccurs.

int find(const vector<int> &list, int value){

int i = 0;

while (i < list.size() && list[i] != value){

i++;}return i;

}

Here we use passing by reference for efficiency

Passing by value vs. Passing by reference

Search through a vector and return the first position where valueoccurs.

void foo(Employee &e){

e.set_salary(1500);}

If we want the change to e be recorded in main we must pass e byreference.

Passing by value vs. Passing by reference

Random numbers

You should know how and why to seed a random number generator.

srand((int)(time(0));

You should know how to get random numbers in a specified range.

The magic formula to get a number in a range [a,b] is

a + rand() % (b-a+1)

but you should know how to adapt this formula if need arises.

Good example of this is the question: Write a single line of code that assignsto an int x a random even number in the range [50,100].

// Get a number in range 25-50 then multiply by 2.int x = 2 * (rand() % (50-25+1) + 25);

Know the syntax!

Declaring a class.

Implementing a class.

Know how to write a constructor, both default and regular.

Know how to overload an operator.

Classes

A class called Team has the following data: name, wins, losses. name is type stringand wins and losses are type int. The class has a default constructor which sets nameto “” and wins and losses to 0 and a regular constructor that takes as arguments aname, wins and losses. It has an accessor get_name that returns the name. It alsohas a mutator called defeats that takes as an argument another Team object and whencalled, it increments wins by one for the instance object and increments losses for theargument object by one. It has an operator > that compares the ratio of wins to losses.

An example of the use of class is:

Team A("Isotopes", 2, 14);Team B("Shelbyvillians", 6, 5);

B.defeats(A); //B's wins are incremented and A's losses areincremented.

if (A > B)cout << A.get_name() << " is the better team.";

elsecout << B.get_name() << " is the better team.";

Class declaration

Class implementation

class Team{public:

Team();Team(string new_name, int new_wins, int new_losses);void defeats(Team &T);bool operator > (Team T) const;string get_name() const;int get_wins() const;int get_losses() const;

private:string name;int wins;int losses;

};

Class implementation

Team::Team(){

name="";wins = 0;losses = 0;

}

Team::Team(string new_name, int new_wins, int new_losses){

name = new_name;wins = new_wins;losses = new_losses;

}

Class implementation

void Team::defeats(Team &T){

wins++;T.losses++;

}

bool Team::operator > (Team T) const{

if (losses==0)return true;

if (T.losses == 0)return false;

if ((double)wins/losses > (double) T.wins / T.losses)return true;

elsereturn false;

}

string Team::get_name() const{

return name;}

Vectors and Arrays

Both arrays and vectors have data of same type.

Access elements with array_name[i] or vector_name[i].

Both have indexes that go from 0 to size-1.

Array size has to be known at compilation time.

Vector size can change because of push_back and pop_back and resizemember functions.

Arrays have no member functions.

Arrays are always passed by reference automatically.

Vectors are passed by value by default, but you can pass by reference ifyou want.

Vectors and Arrays

A function cannot return an array, but a function can return a vector.

2-D arrays are much easier to handle than a 2-D vector.

Arrays are a primitive C++ construct and are more efficient than vectors.

With an array you need to have your own variable that tracks the size of thearray. Often this variable needs to be passed to a function with the array.

Dynamic arrays will not be in the final

Vectors and Arrays

There will without a doubt be questions dealing with arrays and vectors inthe final.

You should know how to traverse through a vector or an array.

You should also know how to go backwards through a vector or an array.

We discussed a lot of important vector/array functions in class. You shouldbe familiar with all of them.

Review the phone book homework assignment.

Vectors and Arrays

Write a bool function that takes as argument two vectors of strings (v1 and v2) andreturns true if v2 contains all elements of v1 in some order and ignoring multiplicities.

bool same_elements(vector<string> v1, vector<string> v2){

bool found=false;for (int i=0; i<v1.size(); i++){

for (int j=0; j<v2.size(); j++){

if (v1[i] == v2[j])found = true;

}if (found)

found = false;else

return false;}return true;

}

2-D Arrays

Write a snippet of code that declares a 10x10, 2-Darray of doubles and then fills it in a following way.For each entry randomly either make the entryequal to its column number divided by row numberor row number divided by column number. Wehave to be careful not to divide by 0, so if thedenominator is 0 just change the denominator to1.

2D-Arrays cont.

srand(time(0));double a[10][10];

for (int i=0; i < 10; i++){

for (int j = 0; j < 10; j++){

if (rand() % 2){

if (j !=0)a[i][j] = double(i)/j;

elsea[i][j] = i;

}else{

if (i !=0)a[i][j] = double(j)/i;

elsea[i][j] = j;

}}

}

2D-Arrays cont.

Output the array:

cout << setprecision(2) << fixed;

for (int i=0; i < 10; i++){

for (int j = 0; j < 10; j++){

cout << setw(5) << a[i][j];

}cout << "\n";

}

File I/O

Once you have your file objects fin and fout and have the file open, readingand writing to a file is just like reading and writing to a console.

You can even use getline(fin, my_string); and all the <iomanip> stuff.

So that leaves only 3 things to know:

1)How do I declare fin, fout objects.2)How do I open a file for reading or writing.3)How do I pass the fin and fout objects to functions

File I/O

We need to #include <fstream>

int main(){ifstream fin;ofstream fout;

string input_file_name;

cin >> input_file_name;

fin.open(input_file_name.c_str());fout.open("output.txt");

...etc....

File I/O

When we use a function to write to a file we should pass our file stream tothe function. Otherwise the function does not know what fout or fin mean!

void output (ofstream &fout, ...other arguments...){// file stuff}

void input (ifstream &fout , ...other arguments...){// file stuff}

The streams should be passed by reference. This is so that we don't loseour place in the file. Remember that fin and fout are keeping track of wherewe are in the file. If we pass by value then the next time we come to thefunction we will have lost our place in the file.

File I/O

Write a full program starting from #include that accomplishes the following task.Ask the user to enter a filename, then open the file for reading. Next, write thecontents of the file in reverse order into a file called backwards.txt. You mayassume that the file that the user specifies contains text separated by spaces. Forexample, if the file contains the phrase:

Hello! How's it going?

The file backwards.txt should have the content

going? it How's Hello!

Hint: First read the contents of the file that the user specifies into a vector.

File I/O

#include<iostream>#include<string>#include<vector>#include<fstream>

using namespace std;

int main(){

string filename;ofstream fout;ifstream fin;string word;

vector<string> file_content;

cout << "Please enter a file name: ";

cin >> filename;

fin.open(filename.c_str());

Continued on next slide...

File I/O

while (fin >> word){

file_content.push_back(word);}

fout.open("backwards.txt");

for (int i=file_content.size()-1; i >=0; i--){

cout << file_content[i];fout << file_content[i] << " ";

}

fout.close(); // optional

return 0;}

We create a pointer to block of given type with the declaration:type* pointer_name;

A pointer stores a memory address.double x = 3.14;double* p = &x;

Alternatively, you can allocate a blank memory block with new.Person *P = new Person;

Dereferencing accesses the pointee’s value directly with *.*p = -42.3;

If you set a pointer equal to array name, it points to 1st element.

string Simpsons[5] = {"Homer","Marge","Bart","Lisa","Maggie"};string* s = Simpsons; //s points to 1st string Homer.

Adding to the pointer moves the pointer through the array.s = s+2; //Now s points to 3rd element Bart

Pointers

Often we will have a pointer to a class

Point *p = new Point(3.2,5.0);

We can access the class by dereferencing the pointer.

cout << "X value is: " << (*p).get_x();

But all the cool kids use the arrow method:

cout << "X value is: " << p->get_x();

Things can look complicated but if you track data types its easy:

people[i]->get_best_friend()->get_name();

Pointers

void fun (int* p, int *&q) {

*p = 100;p += 2;q = p;*q = *(p-1)+*(p+1);cout<<*p<<" "<<*q<<"\n";return;

}

int main(){

int x[4] = {1,2,3,4};int *p1 = x;int *p2; fun(p1,p2); for(int i=0; i<4; i++)

cout<<x[i]<<" ";cout<<"\n"<<*p1<<*p2;return 0;

}

Pointers

#include <iostream>using namespace std;int fun ( int*& p, int* q) {

*p = 12;p = p+3;q = p-1;*q = *(p+1) + *q;*p += 2;cout << "p=" << *p << " q=" << *q << "\n";return (*q)%(*p);

}int main( ) {

int y[5] = {1,2,3,4,5};int* p = y;int *q = new int;*q = fun ( p, q );for (int i=0; i<5; i++)

cout << y[i] << " ";cout << "\np=" << *p << " q=" << *q;return 0;

}

Pointers

7. [20pts] Pointer chasing

What is the output of the following code?

#include <iostream>using namespace std;

void foo (int* &c, int* d){

*(c+2) = 10;int *p = d;*p = 7;d = c;*d = *c + *(c-1);c++; // :)*c = 0;

}

int main(){

int a[5]={1,2,3,4,5};int *p = &a[1];int *q = new int;*q = *(a+4);cout << *p << " " << *q << "\n";foo (p,q);cout << *p << " " << *q << "\n";for (int i=0; i < 5; i++)

cout << a[i] << " ";}

Pointers